Switched reluctance machines have been the subject of increased investigation of late due to its many advantageous characteristics, which make it suitable for use in a wide variety of applications. The switched reluctance machine operates on the basis of varying reluctance in its several magnetic circuits. Referring now to FIG. 1, a diagrammatic cross-sectional view of a prior art 4-phase, 8 stator pole/6 rotor pole switched reluctance motor (SRM) 10 is shown. SRM 10 includes a stator 12 having a plurality of stator poles 14, stator windings 16 (shown only on stator pole A for clarity), and a rotor 18 having a plurality of rotor poles 20. The stator poles 14 appear in pairs: i.e., A A', B B', C C', and D D'. The rotor poles 20 also appear in pairs, but usually in an unequal number as compared to the stator poles pairs. The stator windings 16 associated with diametrically opposite poles (e.g., A and A') are connected in series to form one machine phase. Thus, the windings on poles A and A' are referred to in the art as "phase A" of SRM 10. In the illustrated example, SRM 10 also has phase B, phase C, and phase D.
When a stator phase is energized, the nearest rotor pole pair is attracted towards the energized stator phase, thus minimizing the reluctance of the magnetic path. Therefore, by energizing consecutive stator phases in succession, it is possible to develop constant average torque, and thus rotation, in either direction. Thus, in the specific orientation shown FIG. 1, when phase D is energized, rotor 18 will rotate incrementally clockwise so that the rotor poles 20 nearest stator poles D D' are aligned. If phase A is next energized, rotor poles 20 nearest stator poles A A' will rotate to an aligned position. Accordingly, clockwise rotation of rotor 18 may be accomplished by successively energizing phases A, B, C, D, A, and so on.
The inductance of a winding (known as the "phase inductance") associated with a stator pole pair varies from a minimum when a rotor pole is unaligned with the corresponding stator pole, to a maximum when the rotor pole and the stator pole are aligned. Thus, as a rotor pole sweeps past a stator pole through unaligned-aligned-unaligned positions, the phase inductance varies through minimum-maximum-minimum values. The inductance versus rotor position characteristic is particularly important since for optimum torque production, the current flowing through a stator winding (i.e., known as the "phase current") must be switched on prior to and during the rising inductance period. Further, since positive phase current during the decreasing inductance interval generates negative torque, the phase current must be switched off before this interval to permit the current to decay completely so that no negative torque is produced. Accordingly, an accurate determination of rotor position (e.g., to within 1.degree.) is necessary for precise control of the switched reluctance machine.
Two basic approaches have been practiced by the prior art to determine rotor position: direct methods and indirect methods. Direct methods relate to techniques that directly measure the rotor position, while indirect methods relate to techniques that determine rotor position without measured rotor position information.
The first approach, direct methods, may be further subdivided into "low-resolution" and "high-resolution" techniques. FIGS. 2 and 3 show low speed, and high speed operation, respectively, of a prior art switched reluctance machine equipped with "low resolution" position sensors. "Low resolution" in this context means detection with resolution no finer than one stroke angle, .epsilon., as determined by .epsilon.=(360.degree.)/((number of phases)(number of rotor poles)). In the SRM 10, .epsilon.=15.degree.. Referring particularly to FIG. 2, the motor operation may be characterized by plotting flux linkage .lambda. versus phase current (i). The developed motor torque is proportional to the area inside a .lambda.-i trajectory 22. The position sensors are physically configured so that a minimum torque ripple is achieved, which, for SRM 10, generally corresponds to maintaining energization of a motor phase between rotor angles 37.degree. and 52.degree. with respect to the position of rotor 18 shown in FIG. 1. Thus, as shown in FIG. 1, if the horizontal line through stator tooth C' indicates reference position 0.degree. (i.e., with angular position referenced to increasing values in CCW rotation), then phase C will be energized when the rotor pole having the horizontal line therethrough has rotated CCW 37.degree. from stator pole C', and deenergized at 52.degree. from stator pole C'.
As shown in FIG. 2, this initial energizing point is indicated by point A. The trajectory 22 is then traversed to point B, where the phase current is limited to twenty amperes. Current flow through the energized phase is regulated at twenty amperes as the rotor position moves from 37.degree. to 52.degree. to reach point C. The winding is deenergized at point C, and the current decays to zero amperes. This is represented by path C-D-A.
Conventional motor control, using "low resolution" position sensing technology, maintains this same switching scheme (e.g., 37.degree. to 52.degree. energization) at all rotor speeds including high speed operation shown in FIG. 3. Thus, as shown in FIG. 3, the motor performance is substantially degraded at the higher motor speed, as shown by the significantly reduced area enclosed by .lambda.-i trajectory 22'. Particularly note that due to the stator winding inductance, current build up is slowed. Combined with high motor speed, it is seen that the rotor pole quickly sweeps past the stator pole and the winding must be deenergized before current build up to a satisfactory level occurs. To maintain a .lambda.-i trajectory similar to that depicted in FIG. 2, the turn-on angle, and the conduction angle (i.e., duration) of the energized stator winding must be continuously varied.
"High-resolution" sensing techniques have been implemented using optical encoders or resolvers to provide for the above-mentioned variable control. However, it should be noted that although this approach provides rotor position information sufficiently accurate for satisfactory motor operation over a broad operating range, such direct sensing techniques add cost, and reduce reliability.
The second basic approach, indirect methods, were pursued, in part, due to the shortcoming of the direct techniques. This general approach has taken a wide variety of forms. In one method, rotor position is determined indirectly by measuring winding inductances. Particularly, the inductance is measured by injecting a test signal into a normally de-energized stator winding, or by exciting a special sensing coil wound on a stator. As described above, since the inductance of a stator winding is a function of the rotor position adjacent to the stator pole, the rotor position may be determined indirectly. Another indirect method uses advanced control theory techniques, such as an observer-based state variable model, to estimate rotor position using at least one measured machine operating characteristic (excluding, of course, rotor position itself), such as phase current, phase voltage or the inductance of a deenergized stator winding. Although indirect methods are rugged and less expensive, motor performance is generally poor at the extremes of motor operating range; i.e., low speed high torque and high speed low torque conditions.
Accordingly, there is a need to provide an improved method and apparatus for determining a rotor position of a switched reluctance motor that minimizes or eliminates one or more of the problems as set forth above.